The present invention relates generally to the field of disk drive circuits and more particularly to a method and circuit for driving a hard disk drive motor and the associated control.
A hard disk drive generally includes a stack of rotating disks or platters, a spindle motor which causes the disks to rotate, read/write heads which fly above the surface of the disks, an actuator motor (known as a xe2x80x9cvoice coil motorxe2x80x9d or VCM) which controls the positioning of the read/write heads, power circuitry to provide electrical power to the spindle and voice coil motors, and control circuitry to control the operation of the spindle and voice coil motors.
A read/write head reads data from a disk by sensing flux changes on the magnetic surface of the disk as it passes beneath the read/write head. To synchronize the data being read from the disk with the operation of the data processing circuitry, it is required to carefully control the speed of rotation of the disks. This is usually accomplished by controlling the current delivered to the spindle motor.
Current control can be effected in two ways. The first is pulse width modulation, in which the driving current is modulated by a square wave. Current is delivered to the spindle motor only when the square wave is high. The duty cycle of the pulse width modulation signal therefore determines the average current delivered to the spindle motor.
The other mode of current control is known as linear current control. In this mode, an analog voltage input signal is provided which is proportional to the current to be delivered to the spindle motor. The spindle motor control circuitry processes the input signal and adjusts the level of current delivered by the power circuitry accordingly.
The three-phase brushless motor is one of the most widely used types of spindle motor which has current energizing respective motor coils using a full wave bridge configuration. The bridge includes two power stages for each phase, so typically there are six power stages, each with a power device. Three of the power stages and their power devices are referred to as xe2x80x9clow sidexe2x80x9d stages and devices because they are connected between the motor coil and a sense resistor which is connected to ground. The other three of the power stages and their power devices are referred to as xe2x80x9chigh sidexe2x80x9d stages and devices because they are connected between a power supply and the motor coil.
The power devices are operated as switches in a sequence that allows pulses of current to flow from the power supply through a high side power device, a coil of a first of the three stages, a coil of a second of the three stages, and then through a low side power device to ground. The power device may include a power driver or a FET. This process is repeated in a generally well known manner for the other power devices and coil pairs to achieve three-phase energization from a single, direct current, power supply. The switching, or commutation, characteristics of the power devices are very important in achieving good performance from the motor and other favorable characteristics.
It is necessary to control the current from the motor. One way that has been used is that the motor current passes through a sense resistor to generate a sense voltage. Typically, the sense resistors are small, and the voltage generated by the sense resistors needs to be amplified or gained up. The voltage from the sense resistor is amplified and compared with a reference voltage. When the voltage is above the reference voltage or below the reference voltage, the motor current is either increased or decreased to adjust to a desired level where it matches the reference voltage. However, this method of control of the motor current is unsatisfactory because the voltage gain resulting from the amplification causes offset which is difficult to correct. Furthermore, the voltage comparators are not process independent, and correspondingly, changes in process are not easily compensated for within the amplifier.
The present invention includes a method and apparatus for spindle motor control that is current-based. The present invention uses current in the control of the motor s current. This provides less error. Furthermore, comparing current is more accurate than comparing voltages, and the circuits described allow a process independent comparison of current, which is proportional and a scaled down version of motor current, to a reference current. The current method is not process independent due to its approach in the voltage domain.